3SUM by sorting and two-pointer scan

Also called 3SUM O(n^2) algorithm.

Sort, fix one element, and run a two-pointer scan on the rest, giving O(n^2) time.

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The build computes these levels from the evidence in the repository. Each row says what the next level needs. Read what the levels mean.

Ladder	Level	What the next level needs
Human proof	H0 no proof	add a proof in proofs/ (06.5)
Lean	L0 no Lean part	name a Lean module in the lean field (07)
Visual	V0 none	add viz/scene.ts or a figure (10)
Writing	W0 stub	the section The problem pseudocode in The algorithm complexity claims in the front matter one source with a locator